The invention relates to a circuit arrangement for converting analog signals, particularly PAM signals, into digital signals, particularly PCM signals, and for converting digital signals into analog signals for subscriber stations in a telecommunication setup. A transmitting device transmitting analog signals has an allocated signal output device providing digital signals which occur in pulse frames in cyclical repetition. A receiving device receives analog signals and has an allocated signal accommodation device accommodating digital signals. An analog-digital transducer operating in accordance with the iterative principle is provided comprising a counter, an intermediate register which can be controlled by the counter signals of said counter, a digital-analog converter connected at the outlet side of this intermediate register, and a comparator comparing the analog signals output by the digital-analog converter with the analog signals to be converted into digital signals. The counter controls the output of the counter positions to the intermediate register mentioned with the aid of its output signal and is designed such that it releases the transfer of a digital signal to be converted into an analog signal from an input register of the signal accommodation device to the digital-analog converter in specific counter positions. The counter effectively activates the inclusion of the digital-analog converter for the conversion of an analog signal into a digital signal in specific other counter positions. Additionally, the inputs of an output register of the signal output device are connected to the register stages of the intermediate register. Finally, the outputs of the input register are connected to the setting inputs of the register stages of the intermediate register in accordance with German patent application No. P 2,534,109 corresponding to U.S. Pat. No. 4,056,820 and incorporated herein by reference. Thus, the circuit arrangement in accordance with German patent application No. P 25 34 109 requires only one single analog-digital transducer per subscriber location. This transducer is twice utilized within each pulse frame or slot of the control impulses determining the respective subscriber station. The circuit arrangement is relatively inexpensive as to circuit technique since in addition to the analog-digital transducer which can be of conventional design, only one input register and one output register are required for the conversion of an analog signal into a digital signal and for the output of such a digital signal and also for the accommodation of a digital signal to be converted into an analog signal, and for the conversion of such a digital signal.
If the subscriber stations are a component of a telecommunication setup in which conference calls or connections can be produced between individual subscribers, summation signals are transmitted to the individual subscriber locations. These summation signals are composed of the individual signals of several conference subscribers and, in the case of digitally operating subscriber stations, as it is assumed here, there converted into analog signals.
These summation signals are formed at a central location or, if the subscriber stations are connected to a looped system, are formed at subscriber stations lying therein.
When a summation formation at a central location is developed such that the summation signal is composed of the individual signals of all subscribers taking part in the conference call, the conference set utilized for the summation formation requires less circuitry expense than if summation signals are transmitted to the individual conference subscribers. The summation signals respectively do not contain their own signal share of the receiving subscriber. For closed circuit systems it is natural that the incoming summation signal also contains the previously transmitted self signal share.
As this self signal share in the summation signal received is often perceived as disruptive, an elimination occurs in accordance with a known method (German Auslegeschrift No. 2,445,092) in that the self signal share is not only further transmitted, but is also stored and is subtracted from the summation signal received in the subsequent pulse frame or slot.